Ventricular septal defects are present in approximately 30% of all cases of congenital heart disease. A VSD is a variably sized hole in the septum which divides the left and right ventricles of the heart. If a VSD is present, the pressure difference between ventricles forces part of the exiting left ventricular blood through the defect into the right ventricle, and once again to the pulmonary circulation. Such a condition results in decreased mechanical efficiency of the left heart. For moderate to large VSD's, the volume and pressure on the right side of the heart increase which can result in pulmonary vascular disease(hypertrophic occlusion of the pulmonary arterioles). The study will focus on the fundamental fluid mechanics of VSD's and accomplish two specific aims as follows. (1) To understand the proximal(left ventricular) flow field converging in to the VSD and the interaction between converging flow and crossflowing streams due to systolic ejection. (2) To understand the distal(right ventricular) jet flow field and its interaction with crossflowing streams due to systolic outflow. These aims will be accomplished using computer simulations on the Cray Y-MP C90 using Finite Difference Techniques. The basic understanding will allow the applicant to build on previous theoretical and experimental work in the setting of valvular insufficiency to derive potential VSD quantification techniques.